Comparative Study of Methods to Estimate Elastic-Plastic Dynamic Fracture Initiation Toughness

There has not yet been generally accepted single specimen technique in order to obtain dynamic fracture mechanical parameters. A potential candidate could be the measurement of the acoustic emission (AE). Previous investigations showed that characteristic AE signals appears in the vicinity of crack initiation for the first time. With a specially developed measuring arrangement the method could be compared with simultaneously measuring additional physical parameters at specimens of strongly different levels of toughness and strength. Furthermore the results could be verified by the results of multiple specimens technique. With this technique the dynamic crack resistance curve was determined and the physical crack initiation toughness was deduced by measurement of the stretch zone width. To realize the idea the inverted impact pendulum developed and installed by VTT Espoo/Finland was modified with an AE transducer and a magnetic emission (ME) probe. The inverted impact pendulum design reduces the inertia and oscillation effects in the recorded load curve and allows to measure the crack opening (COD) by a laser scattering light method. In this way, at the same specimen crack opening displacement (COD), AE, and ME could be measured.
Neither of the three single specimen methods can directly detect crack initiation. Except the lower shelf region, were only a tiny amount of ductile crack growth occurs before cleavage, the stable crack initiation could be proven by AE and COD. Good agreement between initiation parameters based on SZW and AE was found for all toughness levels. In some cases a strong scattering of toughness parameters was, however, observed and the ranking did not correspond. COD indicates crack initiation after an amount of macroscopic stable crack growth, which is small but cannot be ignored. The correlation to the SZW is not independent of the toughness; for steels with high toughness the correlation is unsatisfied. Eventually, ME does not reflect stable crack initiation and growth.
Obviously, the physical connection between crack initiation and AE cannot be revealed. The AE method has been proved as very sensible for microscopically active processes, which coincide spatially and chronologically with ductile crack initiation. Thus, it is appropriate to estimate the dynamic fracture mechanical parameter "crack initiation toughness" better than other recently applied methods.